In automatic fruit and vegetable graders the articles of produce commonly are graded, or sorted, according to their colors. The detection of colors usually is done by optical filters and electrooptic means, and the decision as to grading or sorting is based on an electronic evaluation of two or more signals which correspond to different spectra of light reflected from an article being graded.
Many photodetectors desirable for use in produce graders are semiconductor devices that operate with d.c. voltages. As is known, d.c. circuits, particularly high gain d.c. amplifiers which usually are required for use with semiconductor photodetectors, are relatively sensitive in their operation to changes in biasing voltages and to changes in temperature. Consequently, operating stability and calibration are difficult to maintain in produce graders employing d.c. active circuitry such as amplifiers in the low signal level portions of the apparatus. This difficulty is compounded in produce graders that employ multiple electrooptic inspection heads and multiple inspection signal channels to grade respective parallel lines or rows of produce moving on a conveyor belt. In apparatus of this type the calibration of all signal channels should be identical, and if any change of calibration is to be made, i.e., a change of ratio between different color signals in a channel, the change in all channels must be identical. This has been difficult to achieve in automatic produce graders, and much time and effort often is required to achieve and maintain the desired calibration condition for the apparatus.